293T electroporation

Hi there!
I am using a electroporator from biorad (Gene Pulser Xcell) to electroporate 293t cells. I am using the protocol already installed in the machine to do so. I do the electroporation in 100ul dmem (without serum) and after that, I add 500ul complete dmem and tranfer the cells to a plate with complete medium at 37º. I have used 15ugr and 9ugr and both times the cells were dead. I have used electroporation before, but I had no problem with the mortlity (I always had some dead cells, but no so many!).

What do you think could be the problem? Should I use PBS to do the electroporation? Should I wait after the electroporation before I seed the cells in the plate?

Thanks

"He must be very ignorant for he answers every question he is asked" Voltaire